One of the important parameters influencing durability or wear resistance of devices employing a lubricating composition is the effectiveness of phosphorus antiwear or extreme pressure additives at providing devices with appropriate protection under various conditions of load and speed. However, many of the phosphorus antiwear or extreme pressure additives contain sulphur. Due to increasing environmental concerns, the presence of sulphur in antiwear or extreme pressure additives is becoming less desirable. In addition, many of the sulphur-containing antiwear or extreme pressure additives evolve volatile sulphur species, resulting in lubricating compositions containing antiwear or extreme pressure additives having an odour, which may also be detrimental to the environment or evolve emissions that may be higher than increasingly tighter health and safety legislation specifies.
A lubricating composition having the correct balance of phosphorus antiwear or extreme pressure additives provides driveline power transmitting devices with prolonged life and efficiency with controlled deposit formation and oxidation stability. However, many of the antiwear or extreme pressure additives employed have at least one of (i) limited extreme pressure and antiwear performance over a wide range of operating conditions, (ii) limited oxidative stability, (iii) form deposits, or (iv) cause corrosion (for example copper corrosion). In addition, many phosphorus antiwear or extreme pressure additives typically contain sulphur, which results in an odorous lubricating composition containing the phosphorus antiwear or extreme pressure additives. A number of references disclosing antiwear chemistry are discussed below.
U.S. Pat. No. 5,338,470 discloses alkylated citric acid derivatives obtained as a reaction product of citric acid and an alkyl alcohol or amine. The alkylated citric acid derivative is effective as an antiwear agent and friction modifier.
U.S. Pat. No. 4,237,022 discloses tartrimides useful as additives in lubricants and fuels for effective reduction in squeal and friction as well as improvement in fuel economy.
U.S. Pat. No. 4,952,328 discloses lubricating oil compositions for internal combustion engines, comprising (A) oil of lubricating viscosity, (B) a carboxylic derivative produced by reacting a succinic acylating agent with certain amines, and (C) a basic alkali metal salt of sulphonic or carboxylic acid.
U.S. Pat. No. 4,326,972 discloses lubricant compositions for improving fuel economy of internal combustion engines. The composition includes a specific sulphurised composition (based on an ester of a carboxylic acid) and a basic alkali metal sulphonate.
International publication WO 2008/070307 discloses malonate esters suitable as antiwear agents.
International Patent application WO US09/036623 (Filed Mar. 10, 2009 by Najman) discloses a driveline device lubricated with a composition containing a derivative of a hydroxycarboxylic acid and a phosphorus compound that may be either (i) a hydroxy-substituted di-ester of (thio)phosphoric acid, or (ii) a phosphorylated hydroxy-substituted di- or tri-ester of (thio)phosphoric acid.
International publication WO 2005/087904 discloses lubricants containing hydroxy carboxylic acid and hydroxy polycarboxylic acid esters in combination with phosphorus-containing additives. The phosphorus-containing additives include zinc dihydrocarbyldithiophosphates and/or neutral phosphorus compounds, such as trilauryl phosphate or triphenylphosphorothionate. The lubricants are useful in engine lubricants.
International publication WO 2006/044411 discloses a low-sulphur, low-phosphorus, low-ash lubricant composition containing a tartrate ester, or amide having 1 to 150 carbon atoms per ester or amide group. The lubricant composition is suitable for lubricating an internal combustion engine.